Well, they say that different rifles and cartridges sound different, but I honestly couldn't ever tell them apart very well. It all sounds the same - thud, then crack! The best I can say is that the ouder the thud, the bigger (more powder) the cartridge.
To simulate this, get some of those snappers they used to sell - a little powder charge wrapped in paper that kids throw against concrete. Get a wood floor and a sack of potatoes. Simultaneously drop the sack on the floor and throw the snapper. That's what rifle fire sounds like to me through earmuffs.